1. Field of the Invention
This invention relates to a multiplex communication system for detecting an ID code attached to data received from a network and transferring the received data to an instrument corresponding to the detected ID code and to a multiplex communication system which may be used in a broadcast communication system used on cars, for example.
2. Description of the Related Art
In the multiplex communication system used on cars, the types of data (such as the rotation speed of the engine and the speed of the car) are coded, attached to data as an ID code and transferred to a network. Each multiplex communication IC connected to the network detects the ID code, selects only necessary data from the received data and transfers the selected data to the CPU. This operation is effected to transfer only the necessary data among the received data to the CPU and prevent transfer of unnecessary data to the CPU, thereby reducing the load of the CPU. In general, a plurality of types of data are required by each CPU, and an ID code corresponding to data required by each CPU is detected by using a circuit system shown in FIG. 1 or 2 in the prior art.
The circuit shown in FIG. 1 has comparators of a number corresponding to the number of types of necessary data. That is, ID codes which are previously provided for the rotation speed of the engine and the speed of the car, for example, are stored in ID registers 11-1 to 1-4. The ID codes stored in the ID registers 11-1 to 11-4 and ID codes received are respectively supplied to and compared by comparators 12-1 to 12-4. The comparison outputs of the comparators 12-1 to 12-4 are supplied to an OR gate 13 to derive the logical sum thereof. When at least one of the received ID codes coincides with a corresponding one of the ID codes stored in the ID registers 11-1 to 11-4, a coincidence signal indicating the coincidence of the ID codes is output from the OR gate 13.
However, with the construction shown in FIG. 1, the number of ID codes increases with an increase in the number of types of data so that a large number of ID registers 11 and comparators 12 may be necessary, thereby increasing the size of the hardware. Particularly, since the comparators 12 require a relatively large pattern area when the circuit is formed in an IC form, it is disadvantageous when forming the circuit on one chip. Further, since it becomes difficult to change the number of ID codes after the circuit is formed in an IC form, it will be necessary to prepare a large number of comparators for allowance at the time of design. As a result, in many practical cases, redundant hardwares (comparators) may be provided.
The circuit system shown in FIG. 2 is formed to solve the defects of the circuit of FIG. 1 and effect the same operation as the circuit of FIG. 1 by using a comparator, a control circuit and a selection circuit. Like the circuit of FIG. 1, previously attached ID codes are stored in the ID registers 11-1 to 11-4 and the ID codes stored in the ID registers 11-1 to 11-4 are supplied to the selection circuit 14. The selection circuit 14 is controlled by an output of the control circuit 15 and one of the ID codes stored in the ID registers 11-1 to 11-4 is sequentially selected. The ID code selected by the selection circuit 14 and a received ID code are supplied to and compared by the comparator 12. The comparison output of the comparator 12 is output as a coincidence signal indicating the coincidence of the ID codes.
When the coincidence of the ID codes is detected by the comparator 12, the signal is supplied to the control circuit 15 and the succeeding selecting operation of the selection circuit 14 is inhibited.
With the above construction, it is only necessary to use one comparator which requires a large pattern area when an IC is formed so that the pattern area can be made small. However, with the construction shown in FIG. 2, since the number of comparing operations to be repeatedly effected by the comparator 12 becomes larger as the number of ID codes becomes larger, time for comparison becomes long. This is extremely disadvantageous for high-speed transfer, and particularly, for real-time transfer in a system used on a car.
As described above, in the conventional received ID comparison system, the hardware becomes large, and if an attempt is made to solve this problem, the processing time may become long, making it difficult to attain the real-time process.